The disclosure of Japanese Patent Application No. HEI 11-44200 filed on Feb. 23, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a fuel cell system and, more particularly, to a fuel cell system having a polymer electrolyte fuel cell formed by stacking unit cells, each of which has an electrolyte membrane and two electrodes sandwiching the electrolyte membrane.
2. Description of the Related Art
A fuel cell system that determines, based on the humidity in a polymer electrolyte fuel cell and the voltage outputted by the fuel cell, whether the amount of moisture or water contained in electrolyte membranes is insufficient, and a fuel cell system that determines whether the amount of water contained in electrolyte membranes of a fuel cell is sufficient based on an electric current threshold derived by using the amount of change in voltage and temperature as parameters, and on the current outputted by the fuel cell (described in, for example, Japanese Patent Application Laid-Open No. HEI 7-272736) have been proposed. These systems derive an allowable minimum voltage, as a threshold, corresponding to a detected reaction temperature of the fuel cell by using a table that indicates a relationship between the allowable minimum voltage outputted from the fuel cell and the reaction temperature of the fuel cells. If the voltage outputted from the fuel cell is less than the threshold, it is determined that the amount of electrolyte membrane-contained water is insufficient. The systems also derive a maximum electric current value as a threshold by using the amount of change in the voltage outputted from the fuel cell and the temperature of the fuel cells as parameters. If the current outputted from the fuel cell is greater than the threshold, it is determined that the amount of electrolyte membrane-contained water is insufficient. When it is determined that the amount of electrolyte membrane-contained water is insufficient, the systems limit the current supplied to loads.
However, the above-described fuel cell systems have a drawback of reduced precisions in determining the amount of electrolyte membrane-contained water because the amount of electrolyte membrane-contained water considerably changes depending on the fuel cell temperature, the fuel gas pressure, the amount of fuel gas supplied, and the like. Furthermore, when it is determined that the amount of electrolyte membrane-contained water is insufficient, the systems limit the current supplied to loads, so as to protect the electrolyte membranes from damage. However, it is difficult to achieve an appropriate range of the amount of electrolyte membrane-contained water in the systems.
Accordingly, it is an object of the invention to precisely determine a condition of humidification of electrolyte membranes in a fuel cell system. It is another object to perform adjustment such that the condition of humidification remains within a proper range. It is still another object to determine that there is an abnormality in a fuel cell system if the condition of humidification of electrolyte membranes cannot be adjusted to a proper range and to protect the electrolyte membranes from damage when it is determined that there is an abnormality.
To achieve at least one of the aforementioned and other objects of the invention, one aspect of the invention provides a fuel cell system having a polymer electrolyte type fuel cell formed by stacking unit cells, each of which has an electrolyte membrane sandwiched by two electrodes, the system including a fuel gas supplier that supplies a fuel gas to the fuel cell, a fuel gas humidifier that humidifies the fuel gas, a current detector that detects an electric current outputted from the fuel cell, a resistance detector that detects a resistance of the fuel cell, and a humidification condition determiner that determines a condition of humidification of the electrolyte membranes based on the current detected by the current detector and the resistance detected by the resistance detector.
This fuel cell system determines a condition of humidification of the electrolyte membranes based on the current outputted by the fuel cell and the electrical resistance of the fuel cell, which is directly determined by the condition of humidification of the electrolyte membranes. Therefore, the system provides precise determination regarding the condition of humidification of the electrolyte membranes.
Another aspect of the invention provides a fuel cell system having a polymer electrolyte type fuel cell formed by stacking unit cells, each of which has an electrolyte membrane sandwiched by two electrodes, the system including a fuel gas supplier that supplies a fuel gas to the fuel cell, a fuel gas humidifier that humidifies the fuel gas, a current detector that detects an electric current outputted from the fuel cell, a voltage detector that detects a voltage outputted from the fuel cell, a fuel gas supply changer that changes an amount of the fuel gas supplied to the fuel cell, a humidification condition determiner that determines a condition of humidification of the electrolyte membranes based on the current and the voltage detected by the current detector and the voltage detector, respectively, when the amount of the fuel gas supplied is changed by the fuel gas supply changer.
This fuel cell system determines a condition of humidification of the electrolyte membranes based on the current, the voltage, and the amount of the fuel gas supplied to the fuel cell, which is a factor that affects the condition of humidification of the electrolyte membranes. Therefore, the system provides precise determination regarding the condition of humidification of the electrolyte membranes.
Still another aspect of the invention provides a fuel cell system having a polymer electrolyte type fuel cell formed by stacking unit cells, each of which has an electrolyte membrane sandwiched by two electrodes, the system including a fuel gas supplier that supplies a fuel gas to the fuel cell, a fuel gas humidifier that humidifies the fuel gas, a voltage detector that detects a voltage outputted from the fuel cell, a fuel gas supply changer that changes an amount of the fuel gas supplied to the fuel cell, and a humidification condition determiner that determines a condition of humidification of the electrolyte membranes based on a first voltage detected by the voltage detector before the amount of the fuel gas supplied is changed by the fuel gas supply changer and a second voltage detected by the voltage detector after the amount of the fuel gas supplied is changed by the fuel gas supply changer.
This fuel cell system determines a condition of humidification of the electrolyte membranes based on the voltage and the amount of the fuel gas supplied to the fuel cell, which is a factor that affects the condition of humidification of the electrolyte membranes. Therefore, the system provides precise determination regarding the condition of humidification of the electrolyte membranes.
A further aspect of the invention provides a fuel cell system having a polymer electrolyte type fuel cell formed by stacking unit cells, each of which has an electrolyte membrane sandwiched by two electrodes, the system including a fuel gas supplier that supplies a fuel gas to the fuel cell, a fuel gas humidifier that humidifies the fuel gas, a voltage detector that detects a voltage outputted from the fuel cell, a fuel gas supply changer that changes an amount of the fuel gas supplied to the fuel cell, a humidification condition determiner that determines a condition of humidification of the electrolyte membranes based on a first variation of voltages detected by the voltage detector before the amount of the fuel gas supplied is changed by the fuel gas supply changer and a second variation of voltages detected by the voltage detector after the amount of the fuel gas supplied is changed by the fuel gas supply changer.
In this fuel cell system, the voltage detector may detect a voltage of each unit cell of the fuel cell, or may detect a voltage of each module that has at least two of the unit cells. A change in the amount of the fuel gas supplied to the fuel cell affects the condition of humidification of the electrolyte membranes. When the condition of humidification of the electrolyte membranes changes, the change appears in the variation (e.g., variance) of the voltages of the individual unit cells or the individual fuel cell modules.
Therefore, by determining a condition of humidification of the electrolyte membranes based on the voltages of the unit cells or the fuel cell modules, which reflects the condition of humidification of the electrolyte membranes, the system is able to provide precise determination regarding the condition of humidification of the electrolyte membranes.
In the above-described aspects of the invention, a degree of humidification in the fuel gas may be controlled based on the determination made by the humidification condition determiner. In this manner, the condition of humidification of the electrolyte membranes can be adjusted.
Furthermore, an abnormality of the fuel cell system may be detected if the condition of humidification of the electrolyte membranes does not change despite the control of the condition of humidification performed for a predetermined amount of time. In order to provide notification of the occurrence of an abnormality, information regarding the abnormality may be outputted. The operation of the fuel cell system may also be stopped upon detection of an abnormality. Therefore, it becomes possible to protect the fuel cell from damage or the like that can possibly occur during operation in the presence of an abnormality.